Subject: [PATCH v5] drivers/tty: amba: defer probing DMA availability until hw_init

From: [email-masked] (Jorge Ramirez-Ortiz)
Date: Tue, 03 Mar 2015 11:06:32 -0500

On 02/26/2015 11:56 AM, Jorge Ramirez-Ortiz wrote:
> Fix a race condition that happens when device_initcall(pl011_dma_initicall)
> is executed before all the devices have been probed - this issue was observed on
> an hisi_6220 SoC (HiKey board from Linaro).
>
> The deferred driver probing framework relies on late_initcall to trigger
> deferred probes so it is just possible that, even with a valid DMA driver ready
> to be loaded, we fail to synchronize with it.
>
> The proposed implementation delays probing of the DMA until the uart device is
> opened. As hw_init is invoked on port startup and port resume - but the DMA
> probe is only required once - we avoid calling multiple times using a new field
> in uart_amba_port to track this scenario.
>
> This commit allows for subsequent attempts to associate an external DMA if the
> DMA driver was deferred at UART hw_init time.
>
> Signed-off-by: Jorge Ramirez-Ortiz <[email-masked]>
> ---
>  drivers/tty/serial/amba-pl011.c | 95 +++++++++++++----------------------------
>  1 file changed, 30 insertions(+), 65 deletions(-)
>
> diff --git a/drivers/tty/serial/amba-pl011.c b/drivers/tty/serial/amba-pl011.c
> index 8d94c19..4ce98e9 100644
> --- a/drivers/tty/serial/amba-pl011.c
> +++ b/drivers/tty/serial/amba-pl011.c
> @@ -164,6 +164,7 @@ struct uart_amba_port {
>  	bool			using_rx_dma;
>  	struct pl011_dmarx_data dmarx;
>  	struct pl011_dmatx_data	dmatx;
> +	bool			dma_probed;
>  #endif
>  };
>  
> @@ -261,10 +262,11 @@ static void pl011_sgbuf_free(struct dma_chan *chan, struct pl011_sgbuf *sg,
>  	}
>  }
>  
> -static void pl011_dma_probe_initcall(struct device *dev, struct uart_amba_port *uap)
> +static void pl011_dma_probe(struct uart_amba_port *uap)
>  {
>  	/* DMA is the sole user of the platform data right now */
> -	struct amba_pl011_data *plat = dev_get_platdata(uap->port.dev);
> +	struct device *const uap_dev = uap->port.dev;
> +	struct amba_pl011_data *plat = dev_get_platdata(uap_dev);
>  	struct dma_slave_config tx_conf = {
>  		.dst_addr = uap->port.mapbase + UART01x_DR,
>  		.dst_addr_width = DMA_SLAVE_BUSWIDTH_1_BYTE,
> @@ -275,12 +277,19 @@ static void pl011_dma_probe_initcall(struct device *dev, struct uart_amba_port *
>  	struct dma_chan *chan;
>  	dma_cap_mask_t mask;
>  
> -	chan = dma_request_slave_channel(dev, "tx");
> +	uap->dma_probed = true;
> +
> +	chan = dma_request_slave_channel_reason(uap_dev, "tx");
> +	if (IS_ERR(chan)) {
> +		if (PTR_ERR(chan) == -EPROBE_DEFER) {
> +			dev_info(uap_dev, "DMA driver not ready\n");
> +			uap->dma_probed = false;
> +			return;
> +		}
>  
> -	if (!chan) {
>  		/* We need platform data */
>  		if (!plat || !plat->dma_filter) {
> -			dev_info(uap->port.dev, "no DMA platform data\n");
> +			dev_info(uap_dev, "no DMA platform data\n");
>  			return;
>  		}
>  
> @@ -291,7 +300,7 @@ static void pl011_dma_probe_initcall(struct device *dev, struct uart_amba_port *
>  		chan = dma_request_channel(mask, plat->dma_filter,
>  						plat->dma_tx_param);
>  		if (!chan) {
> -			dev_err(uap->port.dev, "no TX DMA channel!\n");
> +			dev_err(uap_dev, "no TX DMA channel!\n");
>  			return;
>  		}
>  	}
> @@ -299,17 +308,17 @@ static void pl011_dma_probe_initcall(struct device *dev, struct uart_amba_port *
>  	dmaengine_slave_config(chan, &tx_conf);
>  	uap->dmatx.chan = chan;
>  
> -	dev_info(uap->port.dev, "DMA channel TX %s\n",
> +	dev_info(uap_dev, "DMA channel TX %s\n",
>  		 dma_chan_name(uap->dmatx.chan));
>  
>  	/* Optionally make use of an RX channel as well */
> -	chan = dma_request_slave_channel(dev, "rx");
> +	chan = dma_request_slave_channel(uap_dev, "rx");
>  
>  	if (!chan && plat->dma_rx_param) {
>  		chan = dma_request_channel(mask, plat->dma_filter, plat->dma_rx_param);
>  
>  		if (!chan) {
> -			dev_err(uap->port.dev, "no RX DMA channel!\n");
> +			dev_err(uap_dev, "no RX DMA channel!\n");
>  			return;
>  		}
>  	}
> @@ -333,7 +342,7 @@ static void pl011_dma_probe_initcall(struct device *dev, struct uart_amba_port *
>  			if (caps.residue_granularity ==
>  					DMA_RESIDUE_GRANULARITY_DESCRIPTOR) {
>  				dma_release_channel(chan);
> -				dev_info(uap->port.dev,
> +				dev_info(uap_dev,
>  					"RX DMA disabled - no residue processing\n");
>  				return;
>  			}
> @@ -362,75 +371,29 @@ static void pl011_dma_probe_initcall(struct device *dev, struct uart_amba_port *
>  					plat->dma_rx_poll_timeout;
>  			else
>  				uap->dmarx.poll_timeout = 3000;
> -		} else if (!plat && dev->of_node) {
> +		} else if (!plat && uap_dev->of_node) {
>  			uap->dmarx.auto_poll_rate = of_property_read_bool(
> -						dev->of_node, "auto-poll");
> +						uap_dev->of_node, "auto-poll");
>  			if (uap->dmarx.auto_poll_rate) {
>  				u32 x;
>  
> -				if (0 == of_property_read_u32(dev->of_node,
> +				if (0 == of_property_read_u32(uap_dev->of_node,
>  						"poll-rate-ms", &x))
>  					uap->dmarx.poll_rate = x;
>  				else
>  					uap->dmarx.poll_rate = 100;
> -				if (0 == of_property_read_u32(dev->of_node,
> +				if (0 == of_property_read_u32(uap_dev->of_node,
>  						"poll-timeout-ms", &x))
>  					uap->dmarx.poll_timeout = x;
>  				else
>  					uap->dmarx.poll_timeout = 3000;
>  			}
>  		}
> -		dev_info(uap->port.dev, "DMA channel RX %s\n",
> +		dev_info(uap_dev, "DMA channel RX %s\n",
>  			 dma_chan_name(uap->dmarx.chan));
>  	}
>  }
>  
> -#ifndef MODULE
> -/*
> - * Stack up the UARTs and let the above initcall be done at device
> - * initcall time, because the serial driver is called as an arch
> - * initcall, and at this time the DMA subsystem is not yet registered.
> - * At this point the driver will switch over to using DMA where desired.
> - */
> -struct dma_uap {
> -	struct list_head node;
> -	struct uart_amba_port *uap;
> -	struct device *dev;
> -};
> -
> -static LIST_HEAD(pl011_dma_uarts);
> -
> -static int __init pl011_dma_initcall(void)
> -{
> -	struct list_head *node, *tmp;
> -
> -	list_for_each_safe(node, tmp, &pl011_dma_uarts) {
> -		struct dma_uap *dmau = list_entry(node, struct dma_uap, node);
> -		pl011_dma_probe_initcall(dmau->dev, dmau->uap);
> -		list_del(node);
> -		kfree(dmau);
> -	}
> -	return 0;
> -}
> -
> -device_initcall(pl011_dma_initcall);
> -
> -static void pl011_dma_probe(struct device *dev, struct uart_amba_port *uap)
> -{
> -	struct dma_uap *dmau = kzalloc(sizeof(struct dma_uap), GFP_KERNEL);
> -	if (dmau) {
> -		dmau->uap = uap;
> -		dmau->dev = dev;
> -		list_add_tail(&dmau->node, &pl011_dma_uarts);
> -	}
> -}
> -#else
> -static void pl011_dma_probe(struct device *dev, struct uart_amba_port *uap)
> -{
> -	pl011_dma_probe_initcall(dev, uap);
> -}
> -#endif
> -
>  static void pl011_dma_remove(struct uart_amba_port *uap)
>  {
>  	/* TODO: remove the initcall if it has not yet executed */
> @@ -1142,7 +1105,7 @@ static inline bool pl011_dma_rx_running(struct uart_amba_port *uap)
>  
>  #else
>  /* Blank functions if the DMA engine is not available */
> -static inline void pl011_dma_probe(struct device *dev, struct uart_amba_port *uap)
> +static inline void pl011_dma_probe(struct uart_amba_port *uap)
>  {
>  }
>  
> @@ -1548,6 +1511,9 @@ static int pl011_hwinit(struct uart_port *port)
>  	uap->im = readw(uap->port.membase + UART011_IMSC);
>  	writew(UART011_RTIM | UART011_RXIM, uap->port.membase + UART011_IMSC);
>  
> +	if (!uap->dma_probed)
> +		pl011_dma_probe(uap);
> +
>  	if (dev_get_platdata(uap->port.dev)) {
>  		struct amba_pl011_data *plat;
>  
> @@ -2218,7 +2184,6 @@ static int pl011_probe(struct amba_device *dev, const struct amba_id *id)
>  	uap->port.ops = &amba_pl011_pops;
>  	uap->port.flags = UPF_BOOT_AUTOCONF;
>  	uap->port.line = i;
> -	pl011_dma_probe(&dev->dev, uap);
>  
>  	/* Ensure interrupts from this UART are masked and cleared */
>  	writew(0, uap->port.membase + UART011_IMSC);
> @@ -2233,7 +2198,8 @@ static int pl011_probe(struct amba_device *dev, const struct amba_id *id)
>  	if (!amba_reg.state) {
>  		ret = uart_register_driver(&amba_reg);
>  		if (ret < 0) {
> -			pr_err("Failed to register AMBA-PL011 driver\n");
> +			dev_err(&dev->dev,
> +				"Failed to register AMBA-PL011 driver\n");
>  			return ret;
>  		}
>  	}
> @@ -2242,7 +2208,6 @@ static int pl011_probe(struct amba_device *dev, const struct amba_id *id)
>  	if (ret) {
>  		amba_ports[i] = NULL;
>  		uart_unregister_driver(&amba_reg);
> -		pl011_dma_remove(uap);
>  	}
>  
>  	return ret;

just following up on this fix (the current driver is broken on my tests).
Is it acceptable at this point?